1. Field of the Invention
The present invention relates to fiber optic probes for spectral analysis. More particularly, the present invention relates to fiber optic probes having endfaces formed for improved light coupling efficiency.
2. Discussion of Background
Fiber optic probes are known for use in the spectral analysis of samples. In a number of designs for probes, the fiber optic probe contains a source fiber and one or more receiving fibers positioned about the source fiber. Upon positioning the probe in a sample, light from the source fiber is directed through the sample to a reflecting surface, where it is reflected back to the probe and collected by the receiving fibers. Comparison of the collected light with the source light, using well known spectral analysis techniques, yields qualitative and quantitative characteristics of the sample.
Variations in fiber optic probes exist in the art to assist in spectral analysis of the sample. For instance, Yafuso et al, in U.S. Pat. No. 4,919,891, disclose an optical fiber having a protective coating surrounding the tip of the fiber. The coating, made of a cross-linked cellulosic material, is permeable to the fluid sample of interest and contains an opaque agent such as carbon black to reduce crosstalk between the fiber and other similar fibers when bundled.
Several fiber optic probes have specific fiber orientations or other directing means to enhance the receipt of light by the collecting fibers. In Kitamura et al, U.S. Pat. No. 4,816,670, an optical measuring head is disclosed having a receiving fiber, a plurality of light-projecting fibers axially disposed about the receiving fiber and a convex lens spaced apart from the ends of the fibers. The lens directs light from the projecting fibers upon a surface adjacent an object to be measured at an angle smaller than a critical angle. Light reflected from the object is focused onto the light receiving fiber.
In U.S. Pat. No. 4,914,284, Halldorsson et al disclose an optical, wide angle head for directionally sensing optical radiation. The device has a plurality of optical fibers, each having a sensor at one end. The sensors are arranged on a mounting plate so that the optical axes of each sensor pass through a common intersection point. Such arrangements are accomplished by bending the optical fibers within the optical head.
However, bending optical fibers in this manner often results in light transmission than is not as focused as if it were being transmitted out of an optical fiber in which the optic axis remains straight. Rather than orienting the receiving fibers to better collect returned light, McGee et al, in U.S. Pat. No. 5,166,756, uses a sapphire rod that abuts the ends of a plurality of optical fibers. The sapphire rod has an endface that prevents specularly reflected light from reentering the probe.
Also, fiber optic probes are known for use in scattered light detection of samples that comprise a translucent or transparent medium containing scattering particles. Such detection includes diffuse reflection, fluorescence and Raman spectroscopy. In this type of analysis, light from the source fiber passes through the sample and is partially scattered by particles having an index of refraction different than that of the sample medium. The scattered light is collected by the receiving fibers and transmitted to a detector for spectral analysis, which determines, among other things, the concentration of the particles in the sample.
Both fiber optic probes disclosed by McLachlan et al (in U.S. Pat. No. 4,707,134 and 4,573,761) have fibers, axially disposed about a central axis, that are bent toward the central axis so that the optic axes of the fibers converge. In U.S. Pat. No. 4,707,134, the probe has a plurality of fibers radially and circumferentially spaced about the axis of a cylindrical housing that is closed at one end by a transparent window. The fibers are bent so that their optic axes converge to intersect one another at a common point on the axis that is adjacent or beyond the outer surface of the window.
In U.S. Pat. No. 4,573,761, a probe used for sensitive Raman analysis is comprised of a bundle of optical fibers in which the collecting fibers are located around the circumference of the transmitting fiber. The fibers are arranged so that the ends of the collecting fibers are tilted slightly towards the center of the probe, preferably at angles less than 45 degrees.
Despite the existence of numerous fiber optic probes, there exists a need for an easily-manufacturable fiber optic probe having an improved light coupling efficiency.